It is known that under certain operating conditions of an internal combustion engine so-called engine knocking can occur. By engine knock is understood the occurrence of shock waves of the fuel-air mixture that are detectable as sound frequency oscillations of the motor, as well as in other ways. Since a heavy thermal loading of the combustion chamber side of the cylinder walls and of the piston usually takes place when there is engine knocking, such that material erosion can occur, efforts are made to avoid knocking, especially since long-repeated knocking can lead to breakdown of the engine. Since, on the other hand, efforts are made to utilize as widely as possible the existing working range of the internal combustion engine, there is a need for detecting knocking of the engine as early and as reliably as possible.
Along with the problem of providing a response to engine knock for detection purposes with a suitable transducer appropriately located, there is also the measurement technology problem of reliably and without interference fishing out the knock signal from the vibrations of the engine picked up by the sensor, in order to provide appropriate control of the engine in dependence upon a knock-detection signal that clearly designates either "knock present" or "knock absent."
A knock detector is disclosed in U.S. Pat. No. 3,440,262 in which the measured knock signal is compared with a predetermined threshhold value signal independent of the motor signal, with a knock recognition signal being provided when the knock signal oversteps the threshhold value. This device has the disadvantage that no knock recognition is produced with reference to the background noise of the engine, but merely by camparison with an external signal independent of the motor.
In U.S. Pat. No. 4,012,942 a knock detector is described in which the measured knock signal is compared with a reference signal that is generated by means of a function generator in response to engine speed. This method has the disadvantage that again the background noise actually occurring in the engine is not taken into account, but merely simulated by a function generator. Furthermore, since the function generator responds merely to engine speed, there is no taking account of the particular manner of operation at the time, the engine control setting and the aging effects which have affected the engine condition progressively.
Finally in German published patent application (OS) No. 29 16 591, a method of determining engine-knock is disclosed in which the peak value of the knock signal is monitored and is set in reference with the preceding peak value that was detected. Furthermore, a provision, among others, is made for performing the measurement only during a so-called measurement window, i.e. during an interval correlated to a particular angular range of the rotation of the crankshaft.
The above-mentioned devices and methods have the disadvantage, however, that extraneous noise in the neighborhood that has nothing to do with engine-knock are not always reliably excluded so that a falsification of the measurement results can take place. This applies particularly for interfering signals that occur by so-called piston bucking and as a result, the amplitude of these interfering signals can provide a substantial covering up of the knock signals.